This application is a National Stage of PCT International Application No. PCT/IB2005/002552 filed Aug. 29, 2005 which claims priority to IT Application No. MO2004A000218 filed Aug. 31, 2004. The entire contents of these applications are incorporated herein by reference.
The invention relates to a method for the control of a wound rotor synchronous motor, particularly but not exclusively suitable for traction of an electric vehicle, for example a forklift truck, and for use in automatic machines, machine tools, lifting machines, industrial robots, and for ship propulsion.
Electric vehicles are known that use drives based on direct-current motors or induction motors or permanent magnet synchronous motors, having different configurations (internal magnets, hybrid motors, etc) coupled with suitable power converters. In these types of drive, the capability of reaching high torque at low speed can be obtained only by injecting a high current into the motor stator windings. Maximum torque is mainly limited by the maximum current obtainable from the electronic converter.
This limitation is due to the high cost per ampere of the electronic power converter and its limited overload capability in terms of peak current and overload time. Oversizing of the power electronic converter is the method that is usually employed to inject high current into the motor and to reach the desired performance at low speed.
This solution often leads to very high costs that can discourage the implementation of drives for heavy electric vehicles powered by a group of standard lead-acid batteries at low voltage (<100V).
The possibility of obtaining constant power operation over a wide speed range has been studied for different configurations of internal permanent magnet synchronous motors (indicated by the acronym IPM-SM standing for “Internal Permanent Magnet Synchronous Motor” in English). In these types of motor, the machine parameters that allow constant power operation up to a high speed do not match the machine parameters required to reach high torque at low speed.
Some solutions have been presented for extending the constant power region without compromising the capability of obtaining high torque at low speed.
The aforementioned solutions include hybrid machines composed of a two-part rotor and comprising a surface magnet part and a reluctance part, field windings positioned in the stator that change the level of excitation produced by the permanent surface magnets, movable parts on both sides of the rotor that short-circuit the flux path, reducing the flux-linkage with the stator produced by the permanent magnets.
In industrial applications requiring constant power operation over a wide speed range an alternative solution to internal permanent magnet synchronous motors (IPM-SM) is represented by wound rotor synchronous motors (indicated as “WR-SM”, the English acronym for “wound rotor synchronous motor”).
In particular, synchronous motors are used for large drives, such as for example ship propulsion and in rolling mill drives. These motors are used because of their high efficiency, high overload capability and good performance in the field-weakening region.
FIG. 1 shows a comparison between the expected performance of a WR-SM and of an IPM-SM in terms of torque and power in function of speed. For the comparison, it is assumed that both motors have the same rated power and the same rated current.
As shown in FIG. 1, a drive based on a WR-SM is suitable for meeting the requirement for high starting torque at low speed that is typical of electric vehicles and for operation at constant power over a wide speed range.
The use of traction systems based on WR-SM in electrical vehicles is proposed in WO9013454.
However, this patent document does not propose a control method at the instantaneous values of a WR-SM.
An object of the present invention is to obtain a method for the control of a wound rotor synchronous motor.
A further object is to obtain a method that enables the operation of a wound rotor synchronous motor to be controlled instant by instant, which wound rotor synchronous motor is arranged to drive an electric vehicle in such a way as to obtain high starting torque and at low operating speed, and constant power operation at high speed.
According to the invention, there is provided a method for controlling a wound rotor synchronous motor, comprising measuring a plurality of electromagnetic entities indicative of the operation of said motor, generating voltage on a stator of said motor to obtain stator currents, further generating further voltage on said rotor to obtain a rotor current, wherein said method further comprises, before said generating and said further generating, processing said entities to calculate references for said stator currents and for said rotor current, said references corresponding to the generation by said motor of the maximum torque obtainable per ampere of stator current supplied.
Owing to the invention, it is possible to obtain a method for controlling a wound rotor synchronous motor that is able to obtain the maximum torque per ampere of stator current in the entire operating field of the motor.
This method is based on the study of instantaneous machine values in linear conditions. The application of the linear model to the real machine operating in the saturation conditions of the magnetic circuit, and therefore in non-linear conditions, is possible by means of a linearization procedure carried out for each operating point. The method uses a flux estimator for estimating machine stator flux. Estimated flux is used to calculate parameters to be inserted into the linear model. By means of the equations of the linear model the optimal supply conditions are identified for each machine operating point, the optimal supply condition complying with operating conditions existing at that point.
Owing to the invention, it is possible to make an electric drive based on the use of a wound rotor synchronous motor that is particularly suitable for the traction of electric vehicles.
Drives based on WR-SM controlled by the method according to the invention are able to generate high torque at low speed by overloading the excitation winding and keeping the stator current at acceptable values, thus avoiding excessive oversizing of the electronic power converter, which supplies the stator.
Furthermore, the possibility of regulating the excitation current enables the machine to operate at constant power up to a theoretically unlimited speed.
These torque-delivering features of the WR-SM machine furthermore meet the electric traction requirements.
The drives disclosed above require an excitation winding on the rotor, brush slip rings for supplying the excitation winding and a direct-current/direct-current chopper for regulating the excitation current. These components are based on consolidated technologies and do not cause a significant increase in the cost of the drives.